Plasmas and ion beams produced from plasma are widely used in the processing, etching and deposition, of semiconductor materials. In spite of this, there are many problems with their use with the more delicate, modern materials, such as the III-Vs, which are damaged by the high energy ions created in the plasma. A novel configuration of a capacitively coupled radio-frequency source was designed and constructed, to overcome these problems. Such a novel source was characterised, and was shown to produce a beam of ions of sufficiently low energy as to avoid causing damage. The source was used to etch GaAs in both Reactive Ion Beam Etching and Chemically Assisted Ion Beam Etching configurations, and Schottky diodes were manufactured on the etched surfaces. Analysis of the characteristics of the diodes showed that a damage free etch had been achieved. The growth of diamond films by Plasma Assisted Chemical Vapour Deposition requires a high plasma density with a low ion energy. To achieve this, commonly microwave plasmas are employed, but there are drawbacks to the use of microwaves, chiefly with regard to "scale-up", and also safety. A development of the novel source used in the early part of this study, led to many of the required plasma characteristics but without the drawbacks. The source was used to grow diamond films, the first successful diamond growth with a capacitive rf source, which were characterised by SEM analysis and Raman spectroscopy.